Increasingly, various types of telephone systems are being used today that terminate in a standard wall-type modular jack, such as the "RJ-11" and "RJ-14" standard modular jacks in North America. Other standard phone jacks are used throughout the world. Other, non-standard phone lines, such as those of high-voltage digital or analog PBX systems, while terminating in a standard phone jack, sometimes provide current capacity that will damage standard equipment. Thus, as people travel with devices that use phone lines, such as portable fax machines and portable computer modems, there is an increasing need for travelers to be able to identify standard telephone lines from non-standard, potentially damaging lines.
There are a variety of conventional telephone line testing devices, most of which are used primarily to determine if a fault condition exists on the telephone line at a given location. Many of these do not provide enough information to the traveler who is seeking to protect his equipment from dangerous current flow.
Such conventional testers are merely simplified devices which use red or green light emitting diodes (LEDs) and resistors to indicate the polarity of the telephone line. One such tester is known and commercially available as "BT-71 Deluxe Phone Line Tester" manufactured by Black Point Products, Point Richmond, Calif., and Radio Shack Catalog Number 43-104 "Phone Line Tester". Other such devices are disclosed in U.S. Pat. No. 4,827,498 to Ross, U.S. Pat. No. 4,626,633 to Ruehl at al., U.S. Pat No. 4,600,810 to Feldman et al., U.S. Pat. No. 4,564,728 to Romano, and U.S. Pat. No. 4,920,555 to Ingalsbe. High current conditions are not tested with these devices. As a result, if one of these devices is used to test a line that has dangerously high current potential, the light emitting diodes (LEDs) of such devices will indicate that power is present and may be illuminated very brightly. But it is impossible for the user of such a device to determine if a dangerous line condition of excessive current potential exists.
More complex examples of telephone line testing circuits have been marketed by IBM and Road Warrior International, and are described as "Modem Saver.TM." devices. Each of these devices employs full-wave rectifiers to supply circuitry for detecting excessive current or voltage, and a reverse polarity indicating yellow LED. The IBM device (disclosed in U.S. Pat. No. 4,533,864) generally measures the current flow capacity of the telephone line, but has temperature dependent trip point characteristics. The Road Warrior device (Road Warrior International part# MODS0001) presents a relatively high line impedance and a complex interaction between the "danger" indicating red LED initiating voltage and a green "safe to use" LED trip-off circuit. Five one-percent resistors are utilized to attempt to match these circuits, but are also plagued with undue temperature dependence of trip thresholds. Neither of these "Modem Saver.TM." prior art devices provide safety provisions against high currents associated with power lines shorting to telephone lines. Further, neither of these devices can be easily modified to provide different trip points for the safe/alarm threshold. What is therefore needed is a portable, low cost, precision current-measuring system to warn modem users when telephone lines present excessive current capacity that may be hazardous to their modems, other equipment, or to the user of such a device. What is also needed is to provide safety against smoke and fire or explosion in the event that such excessive current capacity is detected. A need is for provision of a universal current test instrument that is precise, safe, and low in cost, and which provides a test instrument easily modified for special purpose communication lines or other power sources and special purpose communication or other special purpose equipment in which the safe/alarm threshold is easily adjusted. The ability to indicate and/or test the functioning of the alarm indicating circuitry is also needed in some applications.